I purchased an ST-15 at a gun show in Lakeland in early 2009. It's a 5.56 16" mid-length with a FSB. I wanted to add a drop in quad rail, get rid of the FSB and move to a standard carbine-length handguard setup so I bought a different 16" 5.56 barrel with no FSB from a well-regarded local manufacturer (since shut down) that had one in stock and had it installed along with a 2 rail gas block and removeable front sight. Since that time, the rifle has been blowing primers out of the casing and jamming up the bolt carrier when using 5.56 ammo (not reloaded, brand new winchester 5.56 ammo), so I'm not convinced that the barrel is truly 5.56 spec.

I'd like to swap back to the ST barrel, but I want to free float it, remove the FSB, and also shed some weight and move to a lightweight quad rail of a longer length (was thinking about the 9" or 12" SAR or BAR). Is there any reason this can't be done? What parts will I need to do it and do you guys have them in stock?

I'm guessing this was Anvil Arms? Although they did advertise their 1:9 twist barrels as 5.56, they did not handle M855 (5.56 NATO 62gr) ammo well at all. This lead John to only warrantying the use of M193 ammo and commercial .223 ammo in this rifles. Others in the industry busted on him for misleading advertising. And then mortgaging his company to the hilt for machinery he didn't know how to use to make mil-spec BCG's was even crazier.

History aside, you can do a couple things. First, you could void the warranty on your FSB by taking a dremel to it. This allows you to maintain the solid pin job to the barrel and provide clearance for a longer than 9" rail to cover the FSB. You would still unpin to swap barrel nuts. Second, you could use our micro gas block and micro extension to cover the full gas shoulder (and not leave the taper slots exposed). You would want to dimple the barrel for the set screws on both pieces. This is done by aligning the gas block, tighten one screw to prevent movement, take the other out, use a 5/32 drill bit to create a slight dimple, replace that set screw using red loctite, and do the same process for the other set screws.

The gas blocks and extension should be in stock and ship within a week. You should also get a new crush washer for your muzzle device, as best practice.

A 9" rail will not cover the gas block. A 10" rail will cover the gas block, but not the extension. A 12" rail will cover both and a little barrel, naturally.

If you have any other questions, let us know.

Cheers.

Good guess, it was indeed Anvil Arms. The ammo blowing the primers isn't even 62gr m855. It is new Winchester white box 55gr 5.56.

I'll go with the micro gas block solution and add a 12" BAR. Can I reuse the original gas tube or will I need to source a new one? Also, I assume I'll need the .750 gas block as opposed to the .625? And which size crush washer? 1/2"?

Our original midlength gas tube can be reused. In early 2009, I don't think we had the lightweight midlength barrels yet, so it should be a .750 gas seat. And yes, 1/2" crush washer.

If you want a one piece solution gas block, for a little more, you can also look at the Larue LT202 gas block in .75".